Keep the input in the result of Zstd.write_skippable_frame

[sribalakumar]

Fixes #161.

Change: write the skippable frame, then copy the input after it, and resize to frame size + input size. Also adds RB_GC_GUARD for both string arguments.

New specs (4 examples) in spec/zstd-skippable_frame_spec.rb, under describe 'write_skippable_frame':

• returns the skippable frame followed by the input
• keeps the compressed data decodable by Zstd.decompress
• keeps the compressed data with a magic_variant (magic_variant: 15, checks both read_skippable_frame and Zstd.decompress)
• accepts an empty input

Before

Current main, rspec spec/zstd-skippable_frame_spec.rb: 15 examples, 3 failures.

1) returns the skippable frame followed by the input
   expected: 204
        got: 19

2) keeps the compressed data decodable by Zstd.decompress
   RuntimeError: not a zstd frame (magic not found)

3) keeps the compressed data with a magic_variant
   RuntimeError: not a zstd frame (magic not found)

("accepts an empty input" passes before too, since there's no payload to lose.)

After

15 examples, 0 failures. Full suite: 95 examples, 0 failures.

The README example now works end to end: read_skippable_frame returns "sample data" and Zstd.decompress returns the original data.

Compatibility note

Anyone who worked around the old behaviour by concatenating the frame and the data themselves would now get the data twice, and Zstd.decompress would return the payload twice. If you'd rather not change behaviour, the alternative is to keep it as-is and correct the README instead — happy to switch to that approach if you'd prefer.

Separate thing noticed while testing (not fixed here)

While testing this, I noticed Zstd::StreamingDecompress#decompress returns an empty string when its input begins with a skippable frame — or an empty frame such as Zstd.compress("") — followed by real data; its loop stops at the first frame that produces no output. This is independent of this PR and also happens with a hand-built skippable frame, and Zstd.decompress and Zstd::StreamReader both handle it fine. Happy to open a separate issue for that if useful.